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Date of Origin – 02/24/11
Updated – 02/03/20
CHECKING SPRING BRAKES
Spring breaks must be inspected on a routine basis to insure proper operation. Inspection is recommended a
minimum of every 3 months or 25,000 miles.
WARNING! Always chock wheels to prevent the vehicle from rolling before
performing any brake maintenance.
1. Check overall condition of the foundation brakes including drums, shoes, lining, retainers, return springs, bushings and
rollers. Replace any damaged or worn components per manufacturer’s specications.
2. Check for obvious structural damage to the spring brakes, brake adjusters, or cam shafts and replace any damaged or
worn components per manufacturer’s specications.
3. Hook up appropriate air supply and release parking brakes. Apply and fully release Parking Brake several times while
watching for brake adjuster movement. Adjusters should apply and retract at relatively the same distance for all wheel
positions.
4. To verify equal push rod movement, measure each push rod from the Face of the Air Brake Chamber to the Center of
the Clevis Pin with the brakes Fully Set or Parked. Apply air to the chambers to release the parking brakes and re-mea-
sure all wheel positions. All strokes should be within 1/8” of each other.
5. Applied Stroke at 90 to 100 psi can also be used to measure in a similar way as in step #4. Apply Service Brakes in-
stead of setting Spring (Emergency) Brakes and record before and after push rod measurements. Measured push rod
stroke should not exceed the recommended maximum readjustment limit of 2” for Standard 30/30 chambers and 2 1/2”
for Long Stroke 30/30 chambers.
AUTOMATIC SLACK ADJUSTERS
Effective 10/20/94 all semi-trailers with air brake systems were required to be built with automatic slack adjusters to
assist in maintaining the braking system performance. The automatic slack adjusters should be periodically checked
to insure that they are performing correctly and should be lubricated on a regular schedule or at least every six
months.
The following procedure can be used to check the in-service adjustment of air chamber push rod travel (adjusted
chamber stroke) on trailer air brakes with automatic slack adjusters.
1. Connect an adequate air supply to the SUPPLY glad hand of the trailer’s air system.
2. Increase the air pressure to 100 psi minimum to release the auxiliary spring brake chambers.
3. Determine the size and type of brake chamber you are inspecting.
4. With the brakes NOT APPLIED measure the distance from the bottom of the air brake chamber to the cen-
ter of the large clevis pin on all wheel positions. Record each measurement.
5. Connect a second air supply to the CONTROL glad hand of the trailer’s air system.
6. Increase the air pressure of the second air supply to 85 to 90 psi to apply the service brakes.
7. With the SERVICE BRAKES APPLIED measure the distance from the bottom of the air brake chamber to
the center of the large clevis pin on all wheel positions. Record each measurement.
8. Calculate the adjusted chamber stroke of each brake. Subtract the dimension that was measured in Step
#3 from the dimension that was measured in Step #6. The difference between the two dimensions is the
adjusted air brake chamber stroke. The adjusted air brake chamber stroke must not be greater than 2” for
30/30 chambers and 1 ¾” for 24/20 chambers. If the adjusted air brake chamber stroke is greater than the
dimensions listed, inspect the automatic slack adjuster for wear or damage. See manufacturer’s recom-
mendations for repair or replacement.
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